1. Technical Field
The present invention pertains to methods and apparatus for performing solvent extraction under the control of electrical fields. In particular, the invention pertains to an improved liquid/liquid extraction process and extraction cell structure.
2. Discussion of the Prior Art
The invention described herein is an improvement of the method and system disclosed in U.S. Pat. No. 4,767,515 (Scott et al). The disclosure in that patent is expressly incorporated herein in its entirety.
It is well known in the prior art to extract trace analytes and the like from liquid samples by dispersing the samples in a solvent liquid. Traditional methods for such liquid/liquid extraction of most environmental samples are both labor and solvent intensive, and have high potential for exposure of laboratory workers to solvents and other hazardous substances. Various alternatives, such as solid-phase extraction cartridges or disks, may not satisfactorily cope with difficult aqueous matrices. Improvement of ordinary liquid/liquid extraction techniques, by automating fluid transfers and using electrically enhanced mass transfer, provides opportunity for superior analyte recoveries and more consistent analytical results while reducing solvent and energy consumption and decreasing personnel exposure hazards. An example of a system employing electrically enhanced mass transfer is found in the above-referenced Scott et al patent wherein a method and system for solvent extraction is disclosed. Droplets of the sample are shattered by a high intensity electrical field within the solvent to form a plurality of smaller droplets having a greater combined surface area than the original droplets. The electric field is generated between spaced electrodes disposed within the substantially non-conductive solvent. Dispersion, coalescence and phase separation are accomplished in one vessel through the use of a single pulsing high intensity electrical field under conditions chosen so that simultaneous dispersion and coalescence take place in the emulsion formed in the field. The electrical field creates a large amount of interfacial surface area for solvent extraction when the droplet is disintegrated, and is capable of controlling droplet size and thus droplet stability. These process steps take place in the presence of a counter-current flow of continuous phase solvent. The efficiency of mass transfer during liquid-liquid extraction depends directly upon the amount of the surface area between the two immiscible liquids. Since the solvent (e.g., methylene chloride) is essentially non-conductive, the charged metal electrodes mounted within a solvent-containing region can polarize water droplets, leading to shape distortion, rotation, translation and rupture of the droplets. This greatly increases phase transfer kinetics for extraction of organic analytes from aqueous samples. Concurrently, the electrical field in another region of the cell promotes coalescence of the micron-sized aqueous droplets, thereby promoting phase separation.
In systems such as that disclosed in the Scott et al patent, the application of high voltage pulses to the internal electrodes results in an accumulation of electrical charge between the electrodes. If the number of aqueous solution particles or droplets dispersed between the electrodes becomes sufficiently large, high voltage arcing can occur via droplet "strings" or "chains" subsisting between the electrodes, particularly between the dispersion electrode and the grounded nozzle for injecting the aqueous sample into the solvent. Such arcing causes resistive heating and excessive current flow that could potentially degrade the solvent and dissolve analytes. More specifically, high energy arcing from the internal dispersion electrode is undesirable for a number of reasons. First, if there is any chance that the high voltages employed in the system could in any way change the analytes of interest, it is far more likely to occur as a result of the high energy arc. Second, the possibility of arcing leads directly to the need to detect such arcing and then to regulate or control the high voltage power supply driving the dispersion electrode; both of these functions are expensive and unnecessarily complex. Third, arcing also generates undesirable electrical noise, thereby limiting the utilization of the system for certain applications. It is therefore necessary to operate the electrodes at reduced voltages to avoid these problems, thereby severely limiting the versatility of the system. In addition to the arcing problem, internal electrodes require holes in the cell wall to provide electrical connections for the electrodes. These holes require seals about the connections and even then are subject to leakage. Moreover, space between the cell wall and internal electrodes tends to collect material that could contaminate future samples. Further, since the internal electrodes are in direct contact with the sample or solvent, they must be made of a material which does not in any way react with those liquids. The present invention addresses these problems.
In a system developed to overcome some of the problems described above, it was found convenient to flow extraction solvent and the aqueous sample sequentially out of the extraction cell via the same port at the bottom of the cell after solvent extraction has been completed; that is, the solvent is first caused to flow from the cell, followed by the less dense processed or extracted water layer. In order to permit the sample-carrying solvent to be collected separately from the processed water layer, it is therefore necessary to control a valve in a manner permitting separate collection of the two liquids. The present invention also addresses this problem.